1. Field of the Invention
The present invention relates to a boundary acoustic wave device using a boundary acoustic wave traveling in a boundary between two solid layers and, in particular, to a boundary acoustic wave device having a laminate structure in which one boundary acoustic wave element or one surface acoustic wave element is laminated to at least another boundary acoustic wave element.
2. Description of the Related Art
A variety of surface acoustic wave devices have been conventionally used for an RF filter and an IF filter for cellular phones, a VCO resonator, and a VIF filter for televisions. The surface acoustic wave devices use a surface acoustic wave such as a Reyleigh wave or a first leakage wave traveling along the surface of a medium.
The surface acoustic wave is sensitive to variations in the state of the surface of the medium because the surface acoustic wave travels along the surface of the medium. For this reason, the surface acoustic element is housed in a cavity of a package to protect the wave propagating surface of the medium. The use of the package having the cavity increases the cost of the surface acoustic wave device. In addition, since the package is substantially larger than the surface acoustic wave element, the size of the surface acoustic wave device is increased.
One type of surface acoustic waves travels along a boundary between solid bodies.
Japanese Unexamined Patent Application Publication No. 2-47899 discloses a surface acoustic wave device that uses a Stonley wave, which is one type of surface acoustic wave. FIG. 23 is a sectional view diagrammatically illustrating a surface acoustic wave device 500. An interdigital transducer (IDT) 502 is disposed on a glass substrate 501. A piezoelectric medium layer 503 made of zinc oxide (ZnO) covers the surface of the glass substrate 501 having the IDT 502 provided thereon. The thickness of each of the glass substrate 501 and the piezoelectric medium layer 503 is not less than five wavelengths of the surface acoustic wave, and the glass substrate 501 has a required density and modulus of rigidity. In this arrangement, a Stonley wave travels along the device.
The energy of the boundary acoustic wave concentrates in the boundary between the glass substrate 501 and the piezoelectric medium layer 503 in the surface acoustic wave device 500. Little or no energy of the surface acoustic wave exists on the surface 501a of the glass substrate 501 and the surface 503a of the piezoelectric medium layer 503. The surface 501a and the surface 503a respectively refer to the surfaces of the glass substrate 501 and the piezoelectric medium layer 503 that are opposite from the boundary surfaces thereof. Since there is little or no energy of the surface acoustic wave on the surface 501a of the glass substrate 501 and the surface 503a of the piezoelectric medium layer 503, the characteristics of the device are not sensitive to variations in the state of the surface 501a of the glass substrate 501 and the surface 503a of the piezoelectric medium layer 503. The surface acoustic wave device 500 disclosed in the above-described patent publication does not require a package having a cavity.
A leakage type boundary acoustic wave called a Maerfeld Tournois (MT) wave propagating in a [001]—Si(110)/SiO2/X—LiNbO3 structure is disclosed in a non-patent document entitled “Highly Piezoelectric Boundary Method in Si/SiO3/LiNbO3 Structural Propagation”, 26th EM Symposium, May 1997, pp. 53–58. The boundary acoustic wave of this type has an electromechanical coupling factor k2 that is greater than the Stonley wave. FIG. 24 is a sectional view illustrating a boundary acoustic wave device 510 disclosed in the above-mentioned non-patent document. An IDT 512 is disposed on a Y—X LiNbO3 substrate 511 in the boundary acoustic wave device 510. An SiO2 layer 513 covers the IDT 512. A [001]—Si(100) layer 514 is then disposed on the SiO2 layer 513.
Since the MT wave is a boundary acoustic wave, the boundary acoustic wave device 510 using the MT wave does not require a package having a cavity.
As cellular phones with multiple functions become smaller, flatter, and lighter, miniaturization of electronic components is accordingly required. For example, area requirements of flat RF interstage SAW filters were 3.8 by 3.8 mm2 in 1998, 3.0 by 3.0 mm2 in 2000, and 2.0 by 1.6 mm2 or less in 2002. In the future, area requirements are expected to be less than 1 mm2.
Since the package has no cavity, the above-referenced boundary acoustic wave devices have dimensional advantages over the other devices. However, in the known boundary acoustic wave devices, the dimensions of the element remains unchanged from the known surface acoustic wave devices. Further miniaturization is thus impossible. FIG. 25 is a plan sectional view illustrating electrodes of a 1 GHz band RF interstage filter that is designed in accordance with the known boundary acoustic wave device. As shown, a boundary surface bearing the electrodes is exposed in the sectional view.
As shown in FIG. 25, a number of IDTs 523a–523e, reflectors 524a–524j, electrode pads 525a–525c, and wiring electrodes must be disposed on the top surface of a solid layer 522 made of an LiNbO3 substrate in a boundary acoustic wave device 521 to manufacture the 1 GHz band RF interstage filter. These elements make it difficult to miniaturize the two dimensional area of the boundary acoustic wave device to less than 1 mm2.